Pluto was classified as a planet after its discovery in 1930, but there was always debate about this as it was much smaller than the other planets, and in 2006 it was reclassified as a dwarf planet. This means that it has enough mass to be controlled by the forces of gravity but not to clear the region surrounding it of other objects. It was only in 2006 that any planets began to be described as dwarf ones, as more objects were discovered in space and it became clear that the old categories of planet and satellite weren't enough. Pluto is now the second largest known dwarf planet in the solar system (the largest is Eris, which was also considered a planet when it was first discovered in 2005). Like Eris, Pluto is thought to be about 2,330 km in diameter, but Eris has about 25% more mass.
You can find more information about Pluto here.